Jakob Nielsen has been writing about usability since the dawn of the web. His approaches seemed to go out of fashion for a while, but there has always been a lot of evidence-based common sense in his approach. This post uses OECD data to demonstrate just how limited digital skills are, even in the most advanced countries and draws out the critical point that people who design, build, or even vaguely think about online sites and services are massively unrepresentative.
It is though curious – presumably as a consequence of the way the research was done – that the test tasks described are very work focused. It would be interesting to know if task context and familiarity at least partly countered task complexity.